Wide angle to long focal length zoom lenses have been available for some time. However, there is a constant effort to design such lenses which are more compact, while having a relatively fast aperture. These wide angle to long focal length zoom lenses require different design considerations than a so-called telephoto zoom lens, which, in the thirty-five millimeter format, may be a 70-210 millimeter zoom lens, as exemplified in U.S. Pat. No. 3,817,600.
In the telephoto zoom lenses, it is customary that a rear stationary lens unit have a low telephoto ratio, generally less than one, and the aperture defining iris is included in this rear stationary lens unit. The telephoto ratio is the ratio of the equivalent focal length of the lens or lens lens unit to the front vertex distance (FVD), where the front vertex distance is the distance from the front apex of the lens to the image plane, generally when the lens if focused to infinity. An example of a widely used wide angle to long focal length zoom lens is disclosed in U.S. Pat. No. 4,299,454.
It is common practice to use a basic triplet or variation thereof as a rear lens unit in a zoom lens. The optical powers of a triplet are in a plus-minus-plus configuration separated by small air spaces. The lens units may be subdivided or compounded, but the overall shape remains biconvex. The positive lens units may be and often are meniscus shaped. A triplet, in its simplest form, can be corrected for all primary aberrations, and is particularly well suited for large aperture requirements; however, it is limited with respect to angular coverage because of inherent astigmatism.
In wide angle to long focal length zoom lenses, where the rear lens unit moves to vary the EFL, the iris defining aperture cannot be placed in the rear lens unit. Otherwise, the diameter of the front of the lens would become excessive.
The fourth lens unit thus has to operate with the handicap of having a removed aperture stop. Generally, in wide angle to long focal length zoom lenses utilizing four lens units, a compromise is made in that the aperture stop is positioned closely adjacent the third lens unit and may move with the third lens unit, as disclosed in U.S. Pat. No. 4,299,454, or may remain stationary with the third lens unit, as disclosed in U.S. Pat. No. 4,256,381. If the aperture stop is positioned too far away from the rear lens grouping, it becomes very difficult to control aberration correction without a complex rear lens unit, sacrificing compactness.
A major design constraint of the wide angle to long focal length zoom lens is the size of the rear lens opening. Typically, the clear aperture must be less than thirty millimeters (24.times.36 mm image size). In the wide angle position, light rays pass through the third lens unit at a large angle to the optical axis normally requiring large clear apertures of the elements of the fourth lens unit. If compactness of the lens is not emphasized, then the optical power of the fourth lens unit need not be large and the size of the opening of the fourth lens unit is not so critical. However, in the use of the thirty-five millimeter camera, lens compactness is a primary concern.
Accordingly, the present invention provides a four lens unit wide angle to long focal length zoom lens of plus, minus, plus, plus configuration, having a new and improved design in the fourth lens unit which compensates for the problem of a remote aperture stop, and diameter of the fourth lens unit, and which is relatively simple in a design that contributes to the compactness of the overall lens.